The present invention relates to optical code readers using holographic optical elements, especially optical code readers having input or output optical paths which include one or more electrically switchable systems including holographic optical elements for modifying the optical characteristics of the code reader, for example, to modify the field of view of the code reader.
Optical Code Readers
Optical code readers are known in the prior art for reading various symbologies such as UPC bar code symbols appearing on a label or on the surfaces of an article. The optical code symbol itself maybe a bar code pattern of indicia comprised of a series of bars of various widths spaced apart from one another to bound spaces of various widths, the bars and spaces having different light reflecting characteristics. The readers in scanning or imaging systems electro-optically transform the graphic indicia of a target symbol into electrical signals, which are decoded into information, typically descriptive of the article or some characteristic thereof. Such information is conventionally represented in digital form and used as an input to a data processing system for applications in point-of-sale processing, inventory control and the like. Optical code readers are of two general types: scanning laser beam code readers and imaging code readers.
In the laser beam scanning systems known in the art, the laser light beam is directed by a lens or other optical components along the light path toward a target that includes a bar code symbol on the surface. The moving-beam scanner operates by repetitively scanning the light beam in a line, pattern or series of lines across the symbol by means of motion of a scanning component, such as the light source itself or a mirror disposed in the path of the light beam. The scanning component may either sweep the beam spot across the symbol and trace a scan line across the pattern of the symbol, or scan the field of view of the scanner, or both.
Optical code reading systems also include a sensor or photo detector which detects light reflected or scattered from the symbol. The photo detector or sensor is positioned in the scanner in an optical path so that it has a field of view which ensures the capture of a portion of the light which is reflected or scattered off the symbol. This light is detected and converted into an electrical signal.
Some optical code reading systems are xe2x80x9cretro-reflective.xe2x80x9d In a retro-reflective system, a moving mirror is used to transmit the outgoing beam and receive reflected light. Non-retro-reflective systems typically employ a moving mirror to transmit the outgoing beam and a separate detection system with a wide, static field of view.
Optical codes can also be read employing imaging devices. For example an image sensor may be employed which has a two dimensional array of photo sensor cells which correspond to image elements or pixels in a field of view of the device. Such an image sensor may be a one dimensional (linear) sensor or a two dimensional area sensor such as a charge coupled device (CCD), CMOS device, charge modulated device (CMD) or charge injection device (CID). Associated circuitry produces electronic signals corresponding to a one or two-dimensional array of pixel information for a field of view.
It is known in the art to use a photo detector and objective lens assembly in an imaging optical code reader. In the past, such systems have employed complex objective lenses assemblies originally designed for use in relatively expensive video imaging systems. Such systems may have a single sharp focus and a limited depth of field, which along with conventional aiming, illumination and signal processing and decoding algorithms, limits the versatility and working range of the system.
Other known imaging systems are designed primarily for reading optical code. Such reading systems involve the assembly and alignment of several small parts. These parts may include a lens, an aperture and a 2D optical detector array such as a CCD chip. Such a structure is illustrated, for example, in U.S. patent application Ser. No. 09/096,578 to Correa et al. entitled xe2x80x9cImaging Engine and Method for Code Readersxe2x80x9d filed Jun. 12, 1998 and assigned to Symbol Technologies, Inc. The Correa et al. application is hereby incorporated by reference herein.
Electrically Switchable Holographic Optical Elements
Electrically switchable holographic optical elements (ESHOEs) are known in the art. Such devices may consist of a pair of plates which may be transparent or reflective. A polymer-dispersed liquid crystal material may be located between the plates. One or more interference patterns are formed in the material and define a volume hologram. The ESHOE has optical properties that changes in response to an electrical field applied to the plates. The composition and fabrication of such devices are discussed, for example, in U.S. Pat. No. 5,942,157 to Sutherland et al. entitled xe2x80x9cSwitchable Volume Hologram Materials and Devices.xe2x80x9d
The volume hologram is angle and wavelength selective, which makes a wide variety of applications possible. The hologram can be recorded as a reflection or transmission hologram. The volume hologram may be created by exposing a mix of monomers and liquid crystal located between the plates to intersecting laser beams, giving rise to an interference pattern. Photo-polymerization is selectively initiated by the light to form a matrix of polymer and liquid crystal droplets. When an electric field is applied to the plates the orientation of the liquid crystal molecules changes, resulting in erasing the hologram. When the field is removed, the hologram returns. Application-specific ESHOEs are offered by DigiLens, Inc., of Sunnyvale, Calif.
It has been proposed to use ESHOEs, for example, for projecting images on a projection screen, or providing displays in which the ESHOEs perform simple optical functions commonly associated with traditional optical devices, such as those performed by lenses, prisms and mirrors. It has also been proposed to use ESHOEs in sophisticated optical manipulations such as varying the light intensity with respect to a specific direction. The construction and application of such devices are discussed, for example, in U.S. Pat. No. 6,040,928 to Popovich entitled xe2x80x9cHolographic Desktop Monitor.xe2x80x9d It has been suggested that the DigiLens ESHOEs be used for applications including beam steering, diffractive correction or wavelength selective filtering.
It is an object of the present invention to provide novel applications for volume holograms in optical code readers.
It is another object of the present invention to provide novel applications for ESHOEs in optical code readers.
These and other objects and features of the invention will be apparent from this written description and drawings.
The present disclosure includes various systems and methods employing diffractive or holographic optical elements for improving the performance of optical code readers or providing new functions in such devices.
In one embodiment of the present invention an optical code reading system employs a photo sensor with an array of cells for producing electrical signals responsive to an image directed to said sensor. An optical system sequentially focuses images on the sensor corresponding to at least two different fields of view which may be partially overlapping or completely non-overlapping. The optical system which performs this function includes at least one electrically switchable holographic optical element (ESHOE) for switching between the fields of view. Electronic control circuitry switches the ESHOE to change fields of view. Image data from one or more of the fields of view is selected for decoding in the conventional fashion to extract information from an optical code symbol located in the selected field(s) of view. The system may include processing hardware and/or software for stitching together image portions from at least two of the fields of view to form a composite image with resolution higher than the resolution obtainable with a single exposure of the photo sensor. The system may be used to obtain data sufficient to image a document and to read a bar code which constitutes a portion of the document.
The optical system used to implement this embodiment may include at least one lens through which an input optical path of the system passes and an ESHOE in the optical path. The ESHOE may be formed with a volume hologram which tilts the input optical path of the system when voltage is removed from the plates thereof.
In another embodiment of the present invention an imaging system is adapted for reading an optical code symbol. The system includes a photo sensor having a two dimensional array of cells. An optical system directs images to the photo sensor. The optical system includes at least one ESHOE switchable between two states to vary the optical properties of the images directed to the photo sensor. An electronic switching system, under microprocessor control, may be employed to switch the ESHOE to select an image of the optical code symbol which is readily decodable. Such a system may provide at least two different system magnifications and, thereby, provide a zooming function. Additionally or alternatively, the imaging system may be used to switch between two states which provides different focal distances.
In another embodiment of the present invention, an imager based optical code reading system is equipped with a range finder for determining the distance of an object in a field of view of the system. Such a system employs an image sensor having a primary input optical axis and a field of view. An ESHOE is located on the primary input optical path for selectively tilting the input optical axis of the image sensor. Signal processing circuitry detects a shift in position of an image of the object on the image sensor caused by the tilting of the input optical axis. The distance of the object is determined by triangulation from the detected shift in position. In one embodiment the shift in position of an image of the object is detected by detecting a shift in position of an image of an edge of an optical code symbol on the object. In another embodiment the shift in position of an image of the object is detected by detecting a shift in position of an image of an aiming spot projected by the system onto the object. In either case, hardware and/or software systems responsive to the distance determination may be used to adjust the focal distance of the system.
In another embodiment of the present invention a laser beam source for a laser optical code scanning system employs one or more ESHOEs for selectively modifying an output laser beam. In such systems an ESHOE in an output beam path of the system laser diode is formed with a volume hologram for selectively modifying the laser beam shape in response to a control signal. Control circuitry switches the ESHOE to restore and erase the volume hologram formed in the ESHOE. In one embodiment the volume hologram contains an interference pattern presentation of a cylindrical lens to produce an elliptical laser beam spot. Switching the ESHOE switches the laser beam spot between an elliptical and a circular shape. In another embodiment the ESHOE is reflective and functions as a moving mirror for scanning the laser beam across the optical code.
In another embodiment of the present invention plural transmission mode ESHOES are stacked together to provide multiple, switchable beam shaping functions. At least one of the ESHOEs may be used to control beam ellipticity and at least one other ESHOE may be used to control the location of the narrowest portion of the laser beam waist. A return signal of detected reflections of the laser beam may be monitored to determine the appropriate laser beam shape and the ESHOEs controlled to produce the laser beam shape producing the most effective scanning.
The present invention also includes methods for changing projected messages or aiming patterns from an optical code reader. One such method employs diffractive optical elements formed so that an incident coherent light beam is transformed into at least two different aiming patterns. A first pattern may be projected at a relatively high optical power and a second pattern projected at relatively lower optical power. The power of the coherent light beam directed at the diffractive optical element is varied to vary the appearance of the projected aiming pattern. In one example the power of the coherent light beam is varied to selectively reduce the output power to levels at which the first pattern is visible to a user and the second pattern is too dim to be visible to the user. In effect, at the lower power level, the second pattern is turned off. At the higher power level both aiming patterns appear to form a composite aiming pattern.
In another embodiment of the present invention a color imager is implemented with one or more ESHOEs which sequentially direct different wavelengths of incident light to the cells of the photo sensor.
The foregoing has been provided as a convenient summary of preferred embodiments. However, the invention to be protected is defined by the claims herein and the range of equivalents properly accorded hereto.